Method for producing potassium fluorotantalate crystal being low in oxygen content and method for producing potassium fluoroniobate crystal being low in oxygen content, potassium fluorotantalate crystal bein low in oxygen content and potassium fluoroniobate crystal bein low in oxygen produced by the methods, a

ABSTRACT

The object of the present invention is to provide a method of producing low oxygen-containing potassium fluorotantalate crystals or low oxygen-containing potassium fluoroniobate crystals, the crystals obtained by the production method, and a method of analyzing oxygen contained in these crystals.  
     The method of producing low oxygen-containing potassium fluorotantalate crystals comprises generating recrystallized potassium fluorotantalate crystals by controlled cooling of a saturated solution of potassium fluorotantalate containing hydrofluoric acid as an essential component, collecting the generated recrystallized crystals through filtration, and drying the collected recrystallized crystals in a drying apparatus so as to obtain the crystals, and the production method is characterized in that the saturated solution of potassium fluorotantalate containing hydrofluoric acid as an essential component contains hydrofluoric acid in a concentration of 0.5 mol/l to 10 mol/l.

TECHNICAL FIELD

[0001] The present invention relates to low oxygen-containing potassiumfluorotantalate crystals and low oxygen-containing potassiumfluoroniobate crystals which contain a low content of oxygen, a methodof producing these crystals, and a method of analyzing oxygen which issuitable for analyzing the amount of oxygen contained in these crystals.

BACKGROUND ART

[0002] In recent years, tantalum or niobium powders (hereinafterreferred to as “metal powders” as their generic name) obtained byreduction of potassium fluorotantalate or potassium fluoroniobate havebeen increasingly used in the field of capacitors. In order to produce acapacitor with a high performance, metal powders are required to have ahigh CV value, and therefore various studies have been carried outregarding metal powders.

[0003] Potassium fluorotantalate or potassium fluoroniobate is reducedto metal powders, when it is contacted with fumes such as alkaline metalor alkaline earth metal in an atmosphere with inert gas such as argongas. These metal powders can be used as the electrodes of a capacitorafter sintering. However, if potassium fluorotantalate crystals orpotassium fluoroniobate crystals which are raw materials for metalpowders contain too much oxygen, the amount consumed of alkaline metalor alkaline earth metal which is a reducer in a reduction reactionincreases, and further the reduction reaction does not occur uniformly,and thus it is considered that the CV value cannot be improved.

[0004] CV value is a value used to express performance regarding thecapacitance of a capacitor. The higher the CV value, the smallercapacitor with a larger capacitance can be produced. In recent years, aCV value per weight of metal powders (CV/g) has become large at a levelof several ten thousands to several hundred thousands of CV/g, and it isstill required to obtain a higher CV value. In order to obtain a higherCV value, it is important to reduce leakage current as well asmanufacturing the fine size of metal powders. It is therefore desired toreduce impurities such as metal ions contained in metal powers to aminimum and to produce metal powders containing the lowest level ofoxygen.

[0005] Nevertheless, tantalum and niobium are metals which haveextremely high affinity for oxygen, and it is therefore considered thatthe reduction of the amount of oxygen contained in potassiumfluorotantalate crystals or potassium fluoroniobate crystals would be anextremely difficult technique. Especially, potassium fluorotantalatecrystals have a particularly high affinty for oxygen. The amount ofoxygen contained in potassium fluorotantalate crystals which have beenavailable on the market is over 3% by weight, and the amount of oxygencontained in potassium fluoroniobate crystals is over 3.5% by weight.The production of these crystals, the oxygen content of which is 2% orless by weight, has been considered to be impossible.

[0006] Against this backdrop, as raw materials for metal powders usedfor a capacitor, low oxygen-containing potassium fluorotantalatecrystals or low oxygen-containing potassium fluoroniobate crystals whichcontain a low level of oxygen before a reduction reaction, have beenrequired.

BRIEF DESCRIPTION OF THE DRAWING

[0007]FIG. 1 shows an oxygen detection chart obtained by the method ofanalyzing oxygen of the present invention.

SUMMARY OF THE INVENTION

[0008] The present inventors, in their intensive studies, have devisedthe mother liquor of recrystallized potassium fluorotantalate orrecrystallized potassium fluoroniobate obtained in a recrystallizationprocess, have collected the obtained recrystallized crystals throughfiltration, and have devised a drying process for obtaining the finalproducts, thereby succeeding in reducing the amount of oxygen containedin the products to the lowest level, which has never been achievedbefore.

[0009] A claim relates to a method of producing low oxygen-containingpotassium fluorotantalate crystals, which comprises generatingrecrystallized potassium fluorotantalate crystals by controlled coolingof a saturated potassium fluorotantalate solution comprisinghydrofluoric acid as an essential component, collecting the generatedrecrystallized crystals through filtration, and drying the collectedrecrystallized crystals in a drying apparatus so as to obtain thecrystals; the above method being characterized in that the saturatedpotassium fluorotantalate solution comprising hydrofluoric acid as anessential component comprises hydrofluoric acid with a concentration of0.5 mol/l to 10 mol/l. Another claim relates to a production methodsimilar to the above method, that is, a method of producing lowoxygen-containing potassium fluoroniobate crystals, which comprisesgenerating recrystallized potassium fluoroniobate crystals by controlledcooling of a saturated potassium fluoroniobate solution comprisinghydrofluoric acid as an essential component, collecting the generatedrecrystallized crystals through filtration, and drying the collectedrecrystallized crystals in a drying apparatus so as to obtain thecrystals; the above method being characterized in that the saturatedpotassium fluoroniobate solution comprising hydrofluoric acid as anessential component comprises hydrofluoric acid with a concentration of10 mol/l to 20 mol/l.

[0010] Firstly, a background to the development of these productionmethods will be explained. Oxygen contained in recrystallized potassiumfluorotantalate or recrystallized potassium fluoroniobate refers to twotypes of oxygen such as (1) oxygen derived from water which adsorbs onthe surface of crystals or is taken into the crystal grains on therecrystallization stage (hereinbefore and hereinafter referred to as“oxygen derived from water,”) and (2) oxygen bound and fixed to theinside of the recrystallized crystal grains (hereinbefore andhereinafter referred to as “bound oxygen.”)

[0011] The former water is present mainly in a state where it attachesor adsorbs on the surface of crystals, and it is considered that thiswater is caused by insufficient drying of the crystals, or moistureabsorption by the crystals after drying. In contrast, oxygen bound andfixed to the inside of recrystallized crystal grains is present in theform of K₂TaOF₅ or K₃Nb₂OF₁₁ or the likes. Accordingly, the reduction ofthe above two types of oxygen should be considered.

[0012] The two production methods according to the above describedclaims are directed towards the reduction of the amount of bound oxygen.In order to achieve this object, it is clearly required to control asmuch as possible the generation of K₂TaOF₅ or K₃Nb₂OF₁₁ or the likesexisting in recrystallized crystals in the stage of obtaining therecrystallized crystals. The present inventors have studied whatcomponent in the above saturated solution has the greatest effect on thecontrol of the generation of the above substances. As a result, theyhave found that the concentration of hydrofluoric acid in the abovesaturated solution has the greatest effect. Hydrofluoric acid is anessential component of a saturated solution used to obtainrecrystallized potassium fluorotantalate or recrystallized potassiumfluoroniobate.

[0013] Thus, as a result of intensive studies, the present inventorshave found that, if the hydrofluoric acid concentration is set in therange of 0.5 mol/l to 10 mol/l in a saturated potassium fluorotantalatesolution and if the concentration is set in the range of 10 mol/l to 20mol/l in a saturated potassium fluoroniobate solution, bound oxygen canbe reduced to the lowest level possible, which is 1% or less by weight.The above described minimum hydrofluoric acid concentrations arerequired to achieve the minimum concentration of saturated solution inwhich recrystallization can be carried out for industrial purposes. Ifthe hydrofluoric acid concentration becomes less than the above minimumconcentration, oxyfluorides generate and they are mixed in crystals. Incontrast, if the hydrofluoric acid concentration exceeds the maximumconcentration in each case, a large amount of fine crystals generate,and thereby the amount of water attached to recrystallized potassiumfluorotantalate or recrystallized potassium fluoroniobate crystals to beobtained, begins to sharply increase.

[0014] Secondly, the present invention relates to “the method ofproducing low oxygen-containing potassium fluorotantalate crystals,characterized in that the recrystallized crystals are dried while theatmosphere in the drying apparatus is kept at a temperature of 50° C. to200° C., and that, after the dry process, the recrystallized potassiumfluorotantalate crystals are taken out of the drying apparatus into theoutside air while the difference between the temperatures of therecrystallized potassium fluorotantalate crystals and the outside air iskept at 50° C. or lower so as to control water adsorption on therecrystallized potassium fluorotantalate crystals.” The presentinvention also relates to “the method of producing low oxygen-containingpotassium fluoroniobate crystals, characterized in that therecrystallized crystals are dried while the atmosphere in the dryingapparatus is kept at a temperature of 50° C. to 150° C., and that, afterthe dry process, the recrystallized potassium fluoroniobate crystals aretaken out of the drying apparatus into the outside air while thedifference between the temperatures of the recrystallized potassiumfluoroniobate crystals and the outside air is kept at 50° C. or lower soas to control water adsorption on the recrystallized potassiumfluoroniobate crystals.”

[0015] As stated above, in these production methods, there is discloseda method of adjusting the hydrofluoric acid concentration of motherliquor so as to reduce the amount of bound oxygen in recrystallizedcrystals and also reduce oxygen derived from water. In order to reducethe amount of oxygen derived from water, the recrystallized potassiumfluorotantalate or recrystallized potassium fluoroniobate which wascollected through filtration should be fully dried in the dryingprocess. If only the elimination of water adsorbed on the surface of therecrystallized crystal grains is taken into consideration, the water caneasily be eliminated by increasing the drying temperature to the highestpossible temperature.

[0016] As far as the amount of water adsorbed on the surface of crystalsis concerned, the amount of water adsorbed thereon is affected by thegrain size of crystals. That is to say, if crystals have the sameweight, the smaller the grain size, the larger the specific surface areathat can be obtained. The water adsorption site also becomes broader andthereby the amount of water adsorbed increases. Therefore, where thefluctuations in the amount of oxygen derived from water are reviewed, acomparison should be made among recrystallized crystals having almostthe same grain size distribution.

[0017] However, water is also present in the crystal grains ofrecrystallized potassium fluorotantalate or recrystallized potassiumfluoroniobate. If the elimination of such water is also taken intoconsideration, water should be eliminated slowly, taking enough time.This is because if drying is carried out at an extremely hightemperature, water present in the recrystallized potassiumfluorotantalate or the recrystallized potassium fluoroniobate cannot beeliminated, but the dried surface of the recrystallized crystal grainsis degenerated. Taking these points into consideration, the presentinventors have set the temperature of a dry atmosphere at 200° C. orlower for drying recrystallized potassium fluorotantalate, and at 150°C. or lower for drying recrystallized potassium fluoroniobate.

[0018] In the present invention, the drying apparatus has any type ofatmosphere including an ambient air atmosphere, a vacuum atmosphere, oran atmosphere substituted by inert gas such as argon gas can be applied,as long as it can efficiently eliminate water contained inrecrystallized crystal grains. It is generally considered that atemperature of 100° C. or higher is required to systematically evaporatewater in a short time in an ambient air atmosphere. In a vacuumatmosphere or inert gas atmosphere, a lower temperature can be applied.Even in the case of applying a vacuum atmosphere or inert gasatmosphere, high temperature drying can be applied. However, since suchhigh temperature drying significantly increases the production cost, itis not preferable in terms of cost efficiency. Hence, consideringindustrial productivity, the present inventors have studied appropriatedrying temperature in an ambient air atmosphere, and as a result, theyhave defined 70° C. as the lower limit of drying temperature. If dryingis carried out at a temperature of lower than 70° C., drying time isdrastically increased, and thereby productivity is significantlydecreased. In respect of the structure of the drying apparatus, anydrying apparatus can be applied, as long as it has functions in whichthe readsorption of water removed from recrystallized crystals occurs tothe minimum extent possible and the water can be efficiently eliminatedoutside the drying apparatus.

[0019] Problems occurring in the drying process are not limited to theabove described one regarding drying temperature. In the case of usingpowders having hygroscopicity, since the specific surface area of thepowders is large, when the powders with a high temperature are cooled toa low temperature in the ambient air, they have water absorption.Accordingly, after the drying process, if recrystallized potassiumfluorotantalate or recrystallized potassium fluoroniobate is released tothe ambient air while the substance maintains a high temperature, waterin the ambient air easily adsorbs on the surface of the recrystallizedpotassium fluorotantalate or the recrystallized potassium fluoroniobateagain. Thus, the present inventors have reviewed this problem, and as aresult, they have found that when recrystallized potassiumfluorotantalate or recrystallized potassium fluoroniobate is taken outof the drying apparatus after the drying process, if the differencebetween the temperatures of the substance and the outside air is kept at50° C. or lower, serious water readsorption on the surface of thesubstance can be prevented. Against this backdrop, the production methodof the present invention has been developed. According to the inventivemethod, if a drying temperature or cooling method is optimized, dryingcan be carried out in the ambient air, and low oxygen-containingrecrystallized crystals can be obtained without no cost increase.

[0020] In view of that the amount of oxygen bound to the conventionalrecrystallized potassium fluorotantalate is 1.2% or more by weight andthe amount of oxygen bound to the conventional recrystallized potassiumfluoroniobate is 2.0% or more by weight, the method of the presentinvention provides recrystallized potassium fluorotantalate orrecrystallized potassium fluoroniobate having an extremely reducedamount of oxygen. Therefore, a claim relates to low oxygen-containingpotassium fluorotantalate crystals obtained by the present productionmethod, characterized in that the amount of oxygen fixed on thepotassium fluorotantalate crystals is 1% or less by weight with respectto the total oxygen comprised in the above crystals. Moreover, anotherclaim relates to low oxygen-containing potassium fluoroniobate crystalsobtained by the present production method, characterized in that theamount of oxygen fixed on the potassium fluoroniobate crystals is 1% orless by weight with respect to the total oxygen comprised in the abovecrystals.

[0021] Furthermore, if the production methods according to other claimsare used, not only the amount of bound oxygen but the amount of oxygenderived from water is also reduced, so that the total amount of oxygencontained in recrystallized potassium fluorotantalate or recrystallizedpotassium fluoroniobate can be reduced. Both in the conventionalrecrystallized potassium fluorotantalate and in the conventionalrecrystallized potassium fluoroniobate, the amount of oxygen derivedfrom water is approximately 1.5% by weight. However, according to themethods of the present invention, the amount can be reduced to 1.0% orless by weight. Thus, a claim relates to low oxygen-containing potassiumfluorotantalate crystals used to produce a capacitor obtained by thepresent production method, characterized in that the content of oxygenis 2% or less by weight; and another claim relates to lowoxygen-containing potassium fluoroniobate crystals used to produce acapacitor obtained by the present production method, characterized inthat the content of oxygen is 2% or less by weight.

[0022] It has previously been considered that recrystallized potassiumfluorotantalate or recrystallized potassium fluoroniobate containing theabove-described level of oxygen could not be produced at an industriallevel. However, the use of the above described low oxygen-containingpotassium fluorotantalate or low oxygen-containing potassiumfluoroniobate enables to reduce the consumed amount of alkaline metal orthe like, when these substances are reduced to metal powders. Moreover,the use of these substances also enables to carry out a reductionreaction uniformly, and it is therefore expected that the CV value willbe significantly improved.

[0023] Next, a method of quantifying the amount of oxygen contained inrecrystallized potassium fluorotantalate or recrystallized potassiumfluoroniobate will be explained. The method of quantifying the amount ofoxygen derived from water and the method quantifying the amount of boundoxygen will be explained separately. Water contained in recrystallizedcrystals is considered to attach or adsorb mainly on the surface of therecrystallized crystal grains. Accordingly, the recrystallized crystalsare dried in a drying apparatus which is controlled at a temperature ofapproximately 110° C. so as to reduce the amount of water containedtherein. Thereafter, the value of loss weight is obtained, the amount ofwater is obtained by conversion of the value of loss weight, and theamount of oxygen derived from water is further obtained by conversion ofthe amount of water.

[0024] It is considered that the amount of bound oxygen is determined byX-ray diffraction for analyzing K₂TaOF₅ or K₃Nb₂OF₁₁. However, if thesesubstances are not contained in crystals at a rate of several or more %by weight, detection cannot be carried out, giving poor quantitativeprecision. Therefore, this method has not been used for the qualitycontrol of recrystallized potassium fluorotantalate or recrystallizedpotassium fluoroniobate. That is to say, even if X-ray diffraction wasapplied to recrystallized potassium fluorotantalate or recrystallizedpotassium fluoroniobate, this method could not be used to differentiateproducts which contribute the improvement of performance of a capacitor.

[0025] Against this backdrop, the present inventors have developed amethod of determining the amount of oxygen derived from water and theamount of bound oxygen separately. Thus, a claim relates to a method ofquantifying the amount of oxygen comprised in potassium fluorotantalatecrystals or potassium fluoroniobate crystals, characterized in that itcomprises placing potassium fluorotantalate crystals or potassiumfluoroniobate crystals in a carbon crucible, passing electric currentthrough the above carbon crucible so as to allow the crucible itself togenerate heat due to resistance, thereby heating the potassiumfluorotantalate crystals or the potassium fluoroniobate crystals placedin the above carbon crucible, extracting oxygen comprised in the abovecrystals as carbon monoxide, and quantifying the amount of the obtainedcarbon monoxide so as to quantify the amount of oxygen by the conversionof the amount of the carbon monoxide, the above method being furthercharacterized in that it comprises heating the potassium fluorotantalatecrystals or the potassium fluoroniobate crystals placed in the carboncrucible, measuring a first detection peak corresponding to oxygenderived from water within the range of the carbon crucible temperatureof 100° C. to 200° C., contacting the oxygen derived from water with aplatinum/carbon catalyst which is heated to 1,000° C., and after oxygenwhich is converted into carbon monoxide for detection is not detected,heating the carbon crucible to a temperature of 2,000° C. or higher, andkeeping the carbon crucible at that temperature so as to measure asecond detection peak corresponding to bound oxygen.

[0026] With reference to FIG. 1, this method of quantifying the amountof oxygen will be explained. That is to say, this oxygen quantificationmethod comprises placing potassium fluorotantalate crystals or potassiumfluoroniobate crystals in a carbon crucible, then placing the carboncrucible in a chamber of an oxygen analysis device, and slowly leakinghelium gas that is carrier gas into the chamber for atmospheresubstitution. This method further comprises passing electric currentthrough the carbon crucible so as to allow the crucible itself togenerate heat due to resistance, thereby heating the potassiumfluorotantalate crystals or the potassium fluoroniobate crystals placedin the carbon crucible, decomposing the crystals by heating, andeliminating oxygen contained therein as carbon monoxide. This methodfurther comprises quantifying the thus obtained carbon monoxide with adetector and quantifying the amount of oxygen contained in the crystalsby conversion of the amount of the obtained carbon monoxide.

[0027] In the present invention, while potassium fluorotantalatecrystals or potassium fluoroniobate crystals are being heated, oxygenderived from water and bound oxygen are quantified separately asfollows. First, in order to quantify only oxygen derived from water,potassium fluorotantalate crystals or potassium fluoroniobate crystalsare placed in a carbon crucible and then the temperature of the carboncrucible is raised by resistance heating, thereby heating the crystalsplaced in the crucible. Thereafter, the first detection peak where thetemperature of the carbon crucible is shown in the range of 150° C. to200° C. is measured. The first detection peak means a peak where oxygenis first detected in FIG. 1, which is first confirmed. This firstdetection peak can be detected in a low temperature region, and it isconsidered that this peak corresponds to oxygen derived from water.

[0028] After completion of the detection at the first detection peak,the temperature of the carbon crucible is quickly raised to 2,000° C. orhigher by heating, and then the carbon crucible is kept at thattemperature to detect the second detection peak which corresponds tobound oxygen. It can be said that this second detection peak correspondsto pure bound oxygen because oxygen derived from water is eliminated inadvance and potassium fluorotantalate crystals or potassiumfluoroniobate crystals which are samples are completely decomposed byheating. According to the above described method, the amount of oxygenderived from water and the amount of bound oxygen can be determinedseparately.

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLE 1

[0029] A solution having a liquid temperature of 70° C., a tantalumconcentration of 25 g/l, a potassium concentration of 10 g/l and ahydrofluoric acid concentration of 60 g/l, was used as a saturatedpotassium fluorotantalate solution. This saturated potassiumfluorotantalate solution was poured into a recrystallization bath.

[0030] The rate of cooling the above saturated solution was set at 10°C./hour, and the solution was cooled to a liquid temperature of 10° C.so as to obtain recrystallized potassium fluorotantalate. Thereafter,using a filter press, the generated recrystallized potassiumfluorotantalate was collected through filtration from the solution aftercompletion of the above recrystallization. The collected recrystallizedpotassium fluorotantalate was placed in a drying apparatus with anambient atmosphere and a temperature of 180° C. for the next dryingprocess. After drying for 12 hours, heating is terminated,recrystallized potassium fluorotantalate was cooled to a temperature of40° C., and then the substance was taken out of the drying apparatus. Atthe same time, another drying was carried out in the same manner withthe exception that the drying atmosphere was substituted by nitrogengas.

[0031] With regard to the grain size distribution of the thus obtainedlow oxygen-containing potassium fluorotantalate crystals, 80% by weightof crystals had a size of 0.05 mm to 4.00 mm. Where the amount of oxygencontained in this low oxygen-containing potassium fluorotantalatecrystals was measured, the content of oxygen was 0.271% by weight(oxygen derived from water was 0.016% by weight and bound oxygen was0.255% by weight) when drying was carried out in the ambient airatmosphere, and the content of oxygen was 0.054% by weight (oxygenderived from water was 0.009% by weight and bound oxygen was 0.045% byweight) when drying was carried out in an atmosphere substituted bynitrogen. Considering that potassium fluorotantalate which haspreviously been on the market contained 3% or more by weight of oxygen(oxygen derived from water was 1.5% or more by weight and bound oxygenwas 1.2% or more by weight), it is clear that the present potassiumfluorotantalate crystals contain an extremely low amount of oxygen.

[0032] Analysis of the content of oxygen was carried out as follows. Anempty carbon crucible was previously burned at 240° C. in an analyzer(Oxygen/Nitrogen Analyzer (Horiba Ltd., EMGA-620)). To this carboncrucible, 50 mg of low oxygen-containing potassium fluorotantalatecrystals was placed as samples. Thereafter, the carbon crucible washeated to a temperature of 200° C. followed by measurement of the firstdetection peak. Thereafter, the carbon crucible was heated to atemperature of 2,400° C. followed by measurement of the second detectionpeak. In the meantime, of the oxygen eliminated from lowoxygen-containing potassium fluorotantalate, oxygen derived from waterwas contacted with a platinum/carbon catalyst which was heated to 1,000°C. and it was converted into carbon monoxide. Regarding bound oxygen,oxygen eliminated from low oxygen-containing potassium fluorotantalateby heat decomposition was reacted with carbon constituting the carboncrucible and it was converted into carbon monoxide. Thereafter, eachcarbon monoxide was quantified using an infrared detector. Forcalibration of the device, a calibration sample (JCRM RO21, oxygencontent ratio: 1.05% by weight) was used. A method of measuring theamount of oxygen contained in low oxygen-containing potassiumfluoroniobate, which will be described in the second embodiment, issimilar to the above described method.

EXAMPLE 2

[0033] A solution having a liquid temperature of 70° C., a niobiumconcentration of 40 g/l, a potassium concentration of 50 g/l and ahydrofluoric acid concentration of 300 g/l, was used as a saturatedpotassium fluoroniobate solution. This saturated potassium fluoroniobatesolution was poured into a recrystallization bath.

[0034] The rate of cooling the above saturated solution was set at 10°C./hour, and the solution was cooled to a liquid temperature of 10° C.so as to obtain recrystallized potassium fluoroniobate. Thereafter,using a filter press, the generated recrystallized potassiumfluoroniobate was collected through filtration from the solution aftercompletion of the above recrystallization.

[0035] The collected recrystallized potassium fluoroniobate was placedin a drying apparatus with an ambient air atmosphere and a temperatureof 120° C. for the next drying process. After drying for 12 hours,heating is terminated, recrystallized potassium fluoroniobate was cooledto a temperature of 40° C., and then the substance was taken out of thedrying apparatus. At the same time, another drying was carried out inthe same manner with the exception that the drying atmosphere wassubstituted by nitrogen gas.

[0036] With regard to the grain size distribution of the thus obtainedlow oxygen-containing potassium fluoroniobate crystals, 85% by weight ofcrystals had a size of 0.05 mm to 4.00 mm. Where the amount of oxygencontained in this low oxygen-containing potassium fluoroniobate crystalswas measured, the content of oxygen was 0.583% by weight (oxygen derivedfrom water was 0.229% by weight and bound oxygen was 0.354% by weight)when drying was carried out in the ambient air atmosphere, and thecontent of oxygen was 0.282% by weight (oxygen derived from water was0.111% by weight and bound oxygen was 0.271% by weight) when drying wascarried out in an atmosphere substituted by nitrogen. Considering thatpotassium fluoroniobate which has previously been on the marketcontained 3.5% or more by weight of oxygen (oxygen derived from waterwas 1.5% or more by weight and bound oxygen was 2.0% or more by weight),it is clear that the present potassium fluoroniobate crystals contain anextremely low amount of oxygen.

Industrial Applicability

[0037] Being different from the conventional production methods, theproduction method of the present invention can easily produce lowoxygen-containing containing potassium fluorotantalate crystals or lowoxygen-containing potassium fluoroniobate crystals, which has never beenavailable on the market, without any particular investment in plant andequipment. If these low oxygen-containing potassium fluorotantalatecrystals or low oxygen-containing potassium fluoroniobate crystals areused as electrodes of capacitors, electrodes with low leakage currentcan efficiently be produced, thereby greatly contributing to thestabilization of the quality of capacitors. Moreover, the use of theoxygen analysis method of the present invention enables to analyzeoxygen contained in low oxygen-containing potassium fluorotantalatecrystals or low oxygen-containing potassium fluoroniobate crystals withhigh precision, and it can ensure strong quality assurance of the lowoxygen-containing potassium fluorotantalate crystals or lowoxygen-containing potassium fluoroniobate crystals, when these crystalsare provided on the market.

1. A method of producing low oxygen-containing potassium fluorotantalatecrystals, comprising the steps of recrystallizing potassiumfluorotantalate by controlled cooling of a saturated solution ofpotassium fluorotantalate containing hydrofluoric acid as an essentialcomponent, collecting the formed crystals through filtration, and dryingthe collected crystals in a drying apparatus to obtain a crystalproduct, said method being characterized in that the saturated solutionof potassium fluorotantalate containing hydrofluoric acid as anessential component contains hydrofluoric acid in a concentration of 0.5mol/l to 10 mol/l.
 2. The method of producing low oxygen-containingpotassium fluorotantalate crystals according to claim 1, characterizedin that in the drying step, the collected crystals are dried in thedrying apparatus at an atmospheric temperature of 50° C. to 200° C.,followed by a further step of removing the dried crystals from thedrying apparatus into the outside air so that a temperature differencebetween the dried crystals and the outside air is kept at 50° C. orlower to control water adsorption on the removed crystals.
 3. A methodof producing low oxygen-containing potassium fluoroniobate crystals,comprising the steps of recrystallizing potassium fluoroniobate bycontrolled cooling of a saturated solution of potassium fluoroniobatecontaining hydrofluoric acid as an essential component, collecting theformed crystals through filtration, and drying the collected crystals ina drying apparatus to obtain a crystals product, said method beingcharacterized in that the saturated solution of potassium fluoroniobatecontaining hydrofluoric acid as an essential component containshydrofluoric acid in a concentration of 10 mol/l to 20 mol/l.
 4. Themethod of producing low oxygen-containing potassium fluoroniobatecrystals according to claim 3, characterized in that in the drying step,the collected crystals are dried in the drying apparatus at anatmospheric temperature of 50° C. to 150° C., followed by a further stepof removing the dried crystals from the drying apparatus into theoutside air so that a temperature difference between the dried crystalsand the outside air is kept at 50° C. or lower to control wateradsorption on the removed crystals.
 5. Low oxygen-containing potassiumfluorotantalate crystals produced by the method according to claim 1,characterized in that the amount of oxygen fixed to the potassiumfluorotantalate crystals, among the total oxygen contained in saidcrystals, is 1% or less by weight with respect to the crystal product.6. Low oxygen-containing potassium fluorotantalate crystals produced bythe method according to claim 2, characterized in that the content ofoxygen is 2% or less by weight with respect to the crystal product. 7.Low oxygen-containing potassium fluoroniobate crystals produced by themethod according to claim 3, characterized in that the amount of oxygenfixed to the potassium fluoroniobate crystals is, among the total oxygencontained in said crystals, 1% or less by weight with respect to thecrystal product.
 8. Low oxygen-containing potassium fluoroniobatecrystals produced by the method according to claim 4, characterized inthat the content of oxygen is 2% or less by weight with respect to thecrystal product.
 9. A method of quantifying the amount of oxygencontained in potassium fluorotantalate crystals or potassiumfluoroniobate crystals, comprising the steps of placing potassiumfluorotantalate crystals or potassium fluoroniobate crystals in a carboncrucible, passing electric current through said carbon crucible so as toallow the crucible itself to generate heat due to resistance, heatingthe potassium fluorotantalate crystals or the potassium fluoroniobatecrystals placed in said carbon crucible, extracting oxygen contained insaid crystals as carbon monoxide, quantifying the amount of the carbonmonoxide, and converting the amount of the carbon monoxide to the amountof oxygen, characterized by: heating the potassium fluorotantalatecrystals or the potassium fluoroniobate crystals placed in said carboncrucible within a temperature range of 100° C. to 200° C., so as tomeasure a first detection peak corresponding to oxygen derived fromwater; heating the carbon crucible to a temperature of 2,000° C. orhigher, after no more oxygen has been detected within the temperaturerange of 100° C. to 200° C. in the carbon crucible; and keeping thecarbon crucible at that temperature so as to measure a second detectionpeak corresponding to bound oxygen.